Page 36 - Application Handbook - TOC
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Sum parameter – Total Organic Carbon




                                         TOC – Determination in sodium
                                         hydroxide solution
                    SCA-130-305


            Organic contaminants in basic chemicals may         Several mL of ultrapure water were placed in
            lead to impurities in the products. Therefore,      a 50 mL volumetric flask. Subsequently, 5 mL
            quality control of the reactants is necessary.      of the concentrated sodium hydroxide was
                                                                pipetted into the flask. Finally, concentrated
                                                                sulfuric acid was added until the solution has
                                                                reached a pH < 2. The flask was then filled
                                                                with ultrapure water up to the mark. The
                                                                addition of sodium hydroxide, as well as the
                                                                addition of sulfuric acid to the water must be
                                                                done with the utmost care and caution, as a
                                                                violent chemical reaction occurs.

                                                                 Compound                             Conz.
                                                                  (Concen-           Dilution          [%]
                                                                   tration)
            The TOC determination in sodium hydroxide                              1 : 10 diluted
            can lead to various problems. The catalyst                                               approx.
            and the  combustion  tube wear out very                Sodium       (5 ml / 50 ml) add.     5%
            rapidly. This, in turn, will lower the sensitivity    hydroxide     1-2 drops Sulfuric
            at an equally fast rate and leads to very poor         solution       acid until pH<7    + Sulfuric
            reproducibilities.                                      (50%)      (Note: generation of    acid
                                                                                      heat)
            NaOH can also absorb CO 2 from the
            environment. As air contains approximately
            400 ppm  CO 2, direct TC determination in
            sodium hydroxide can lead to much higher            Calibration of the TOC-L system was carried
            values. The NPOC  method is, therefore,             out using  the automatic dilution function
            recommended for the determination of                within the range of 0.5 mg/L to 10 mg/L.
            organic contaminations in sodium hydroxide.
            The sample should also be diluted as much
            as possible.

            In the present case, a 50% sodium hydroxide
            solution was analyzed. The purity criterion
            and the required limit of detection was < 10
            mg/L.

            ■ Sample Preparation                                            Fig. Multi-point calibration
            The sample was first  manually diluted to a                      with dilution function
            ratio of 1:10 with ultrapure water and a
            suitable amount of acid.
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